Maybe we don't want to come in on the tail end of a run especially if the gear we're in search of comes from a boss that's already dead. Or maybe we've killed the last boss already and need a run that will let us get the first two. There are any number of reasons but regardless, we shouldn't be penalized for 30 minutes for wanting a fresh run.
We used to be able to see how many bosses were down already and we could make an informed decision about whether or not we wanted to join. Maybe there's been some discussion explaining why the change was made but, I'm sorry, it was a dumb idea that takes away player choice.
Please bring back the raid preview.